1. Field of the Invention
Disclosed herein are novel electrochromic materials comprising doped metal oxide-containing particles having a high contrast and methods of making and using such materials, e.g., in novel electrochromic devices, wherein at least some of such devices are characterized by a substantially linear relationship between an amount of electrical charge stored and a voltage between terminals of the electrochromic device.
2. Description of the Prior Art
Doped tin oxides are known among metal oxides for their relative transparency and high electrical conductivity. These properties are advantageously employed in a variety of electro-optical applications, e.g., providing transparent conductive coatings on particles or surfaces. One such application is the fabrication of transparent electrodes on electrochromic display devices which typically have an electrolyte material in contact with an electrochromic material so that an electrochromic effect is generated when an electric potential is applied across the interface of the two materials. When electrodes are provided on both sides of the materials, e.g., in sandwich-like structure, the electrode on at least one side of the display laminate needs to be relatively transparent to permit observation of the electrochromic effect. In such devices typical electrochromic materials include tungsten oxide, Prussian Blue, polyaniline and viologens. Transparent electrodes have been fabricated by vapor deposition of doped tin oxide coatings on glass or plastic substrates.
Although doped tin oxides have been employed as transparent conductors in electrochromic devices, it appears that the possibility that doped tin oxide might be useful as a practical electrochromic material has not been discovered. For instance, Orel et al. reported in the Journal of the Electrochemical Society, Vol. 141, page L127 (1994) that a film of ATO exhibited a change in light reflectance between a reduced and oxidized state of less than 5%, which corresponds to a contrast ratio (as defined hereinbelow) of less than 1.05. Because such a change in color is not readily discernable to the typical human eye, it has not been recognized or discovered that doped tin oxides have useful electrochromic properties.
A variety of dopants are used to make conductive metal oxides, some of which, e.g., fluorine-doped tin oxide and tin-doped indium oxide (ITO), have not heretofore been known to produce materials exhibiting any useful electrochromic effect. Similarly, ATO, when provided in film form, also does not exhibit any useful electrochromic effect.
Furthermore, in prior art electrochromic devices, Faradaic processes, e.g., metal deposition, takes place, causing the prior art devices to have a non-linear relationship between charge and voltage. These Faradaic processes cause charge to increase rapidly after a certain threshold voltage value is reached, as discussed by B. Conway in The Electrochemical Society Proceedings, Vol. 95-29, "Electrochemical Capacitors," p. 15, a phenomenon that increases the difficulty in controlling electrochromic displays, because neither charge nor color is a linear function of voltage in such a display. It would be desirable to provide an electrochromic display free from electrical effects caused by Faradaic processes, so that an amount of charge injected into the display would be directly proportional to the applied voltage over a useful voltage range. To the extent that the color change of the display would also be directly and linearly related to the amount of injected charge, the color change, as well, would be linearly related to voltage, which would permit the use of simplified display control circuitry.